Abstract: A novel carbide free bainite/martensite (CFB/M) high strength steel of 1500MPa grade is micro-alloyed with Nb in order to refine the microstructure and improve the toughness. After the steel containing Nb was forged, coarse microstructure and strong transmissibility of structure were found. It is suggested that the large numbers of precipitates of Nb(CN) in the steel makes the structure transmissibility more serious. After solution treatment at 1200°C, both complete annealing twice and isothermal treatment at 600°C twice can eliminate the structure transmissibility and refine the structure. Investigation on refinement of cycle phase transformation shows that excellent effect of the grain refinement is obtained when the heating velocity exceeds some threshold. In addition, the temperature of austenitizing is of little influences on the size of refined austenite grain.

Abstract: The new generation TMCP process based on ultra fast cooling has recently developed rapidly. In order to develop the low yield ratio constructional steel, the effects of cooling finishing temperature on microstructure and yield ratio of steels were studied primarily. The results show that the high strength and low yield ratio can be achieved by multiphase including M-A hard second phase and bainite ferrite soft matrix. When UFC final temperature is 521°C, the uniform M-A phases with volume fraction of 22.5% are distributed on bainite ferrite matrix, and then the yield strength is 570MPa, tensile strength 760MPa, yield ratio 0.75, and percentage elongation 22% with the Charpy impact energy 284J at -40°C. All these indexes come up to the relevant standards. The steel with lower yield ratio can be subjected to larger plastic deformation before the necking instability.

Abstract: Microstructures in the weld heat affected zone (HAZ) can cause a decrease in fracture toughness, and evaluating the effect of microstructures on fracture toughness is helpful in understanding the cause of the fracture toughness decrease. In this study, micro-sized tensile specimens were sampled from base metal and HAZ, and the mechanical properties and fracture behaviours of different steel microstructures were directly investigated by micro-sized mechanical testing.

Abstract: The variation of microstructure and their effect on the mechanical properties of heavy plate for ship hull treated by quenching in full austenite region at 910°C once or twice and tempering, or quenching in full austenite region and quenching in intercritical austenite region at 840°C again and tempering were studied with SEM and TEM. It was found that the strength and low temperature toughness of the steel were higher after double quenching in full austenite region and tempering than the steel which was quenched once and tempering, because smaller bainite inter-lath spacing, finer grains were obtained. The steel treated by intercritical quenching at 840°C after quenching in full austenite region and tempering showed the best combination of mechanical properties. This can be attributed to comprehensive effect of the uniform distribution of ferrite and bainite, stronger grain refining effect caused by formation of ferrite.

Abstract: In this paper, ultra-high strength dual phase steel was investigated. Thermomechanical processing was conducted by using a laboratory hot rolling mill. The results have shown that the main transformation products at three different kinds of thermomechanical processing were ferrite, bainite, and small amounts of martensite. Laminar cooling led to ferrite grain refinement. The mechanical properties of specimen 1 which was controlled cooling after a relative lower temperature rolling are much higher than that of specimen 2. The presence of martensite islands and precipitates contributed to the enhancement of strength of the present steel. And the presence of retained austenite resulted in higher toughness. As a result, these specimens exhibited satisfactory mechanical properties.